1. Field of the Invention
The present invention relates to an improvement in a method for growing semiconductor epitaxial layers by a liquid phase epitaxial growth.
More particularly, the present invention is concerned with a method for growing semiconductor epitaxial layers on a semiconductor substrate for sequentially forming liquid phase epitaxial growth layers of opposite conductivity types to form a p-n junction inbetween.
2. Description of the Prior Art
A liquid phase epitaxial growth method is generally being used in forming semiconductor epitaxial growth layers such as GaAs, GaP or GaAlAs on a III-V compound semiconductor substrate, and is a fundamental method of manufacturing semiconductor lasers or light emitting diodes.
Hitherto, there have been two sequential epitaxial growth methods for forming semiconductor epitaxial growth layers with p-n junction inbetween on a semiconductor substrate. A first method uses two solution tubs. A first solution tub contains a first solution comprising a solvent such as Ga, In or Sn, a solute of III-V semiconductor such as GaAs, GaP or GaAlAs and an impurity for determining a conductivity type. A second solution tub contains a second solution having similar contents but containing an opposite type impurity to that of the first solution. The epitaxial growth is made by sequential contacts of the first and the second solutions on the substrate.
A second method uses one solution tub containing a semiconductor solution of one conductivity type and after forming a first epitaxial layer by contacting the solution of a first conductivity to the semiconductor substrate, an opposite type impurity is given from gas phase or solid phase to the solution thereby compensating the first conductivity and forming a second epitaxial layer.
FIG. 1 shows an apparatus utilizing the first method using two tubs, wherein a slider 10 is slidably disposed on a holder 5 having a recess 51 for holding a semiconductor substrate 6 therein. The slider 10 comprises two solution tubs 1 and 3, which respectively contain solutions 2 and 4 of opposite conductivity types relative to each other. In such an apparatus, sequential epitaxial growth is made by firstly putting the first solution tub 1 on the substrate 6 and then, by moving the slider 10 to the right, placing the second solution tub 2 on the substrate 6. The abovementioned first conventional method using two tubs has a problem, such that, although impurity concentration controls are easy due to separate semiconductor tubs, the two tubs require a large volume of solution and hence are disadvantageous from both a mass-production and economic view point.
FIG. 2 shows an apparatus utilizing the second method using a single tub, wherein a slider 10 is slidably disposed on a holder 5 having a recess 51 for holding a semiconductor substrate 6 therein. The slider 10 comprises only one semiconductor solution tub 8 which originally contains a semiconductor solution 7 having an impurity therein. In such an apparatus, sequential epitaxial growth is made by firstly contacting the solution 7 of the tub 8 to the substrate 6 thereby forming first epitaxial layer thereon, and secondly adding an opposite type impurity to the solution by, for example, a gas phase, thereby compensating the original impurity and forming a second epitaxial layer of a second conductivity type on the first epitaxial layer. The abovementioned second conventional epitaxial growth method using a single tub has a problem such that, although the mass-productivity and economical aspects are good, a considerable amount of the second impurity is necessary in order to compensate the first impurity contained in the solution. Therefore, the second epitaxial layer necessarily contains a high concentration of impurity, resulting in considerable crystal imperfection and poor light output. The light emitting diode made in accordance with the second method using a single tub has a smaller light output in comparison with that made in accordance with the first method using two tubs.
FIG. 3 is a time graph plotting temperature change of the semiconductor substrate and solution in degrees Celcius against time in an actual manufacturing process for a GaP green light emitting diode utilizing the apparatus of FIG. 2. Details of the manufacturing process are as follows:
A semiconductor solution in the tub 8 is prepared by placing the following material in the tub 8:
Ga (as solvent): 10 g PA1 GaP (polycrystalline; as solute): 350 mg PA1 Te (as impurity of n-type conductivity): 100 .mu.g.
The abovementioned components are preliminarily heated in the tub 8 at the position as shown to the temperature of T.sub.1 =1020.degree. C. for the time period of .tau..sub.1 =30 min., namely from the time t.sub.1 to the time t.sub.2, so that the material sufficiently dissolves in the solution. Nitrogen as a recombination center is introduced from a carrier gas comprising NH.sub.3 and H.sub.2 from the time t.sub.1 on. After the preliminary heating of the solution for the time period of .tau..sub.1, the solution is contacted with a GaP substrate 6 from the time t.sub.2 to the time t.sub.3, namely for the time period of .tau..sub.2 =20 min., by sliding the slider 10 to the right. The substrate and the solution are then cooled at a predetermined cooling rate from the time t.sub.3 to the time t.sub.4, namely for the time period of .tau..sub.3 =25 min., down to the temperature of T.sub.2 =920.degree. C., thereby growing the first epitaxial layer of n-type conductivity. After forming the n-type first epitaxial layer, zinc as a p-type impurity (acceptor) is introduced from the vapor phase into the semiconductor solution 7, in an amount such as to compensate the donor of Te. The time from t.sub.4 to t.sub.5 is the zinc addition and subsequent aging time. Then, for a time period of .tau..sub.4 from the time t.sub.5 to the time t.sub.6, the substrate is cooled to T.sub.3 =800.degree. C. thereby forming a second epitaxial growth layer of p-type conductivity on the aforementioned first epitaxial growth layer of n-type conductivity.
A light emitting diode produced in accordance with the abovementioned conventional single tub process contains impurities Te and N in its n-type first epitaxial layer and Te and Zn in its p-type second epitaxial layer. The light output of such a light emitting diode is low, its efficiency is as low as 0.05 to 0.1% due to the high impurity concentration as a result of the compensation in the p-type epitaxial layer.
As has been described, the conventional methods of liquid phase growth have the problems of lacking economical mass-productivity and in inferior product characteristics.